The Iron Bore Head is a component from the Railcraft mod. This component is operate the Tunnel Bore properly. This is the piece that allows it to break blocks, like the Diamond Bore Head and the Steel Bore Head. This Bore Head has a durability of 1500, and does not allow the Tunnel Bore to mine Obsidian.
It goes in the Head slot of the Tunnel Bore's GUI. It will begin mining as soon as it has a head, tracks, fuel, and ballast.